per-config build output using VPATH

based on https://make.mad-scientist.net/papers/multi-architecture-builds/

.github/workflows/rust.yml

@@ -36,9 +36,9 @@ jobs:
         run: output=$(git status --porcelain) && [ -z "$output" ]
 
       - name: build example -- glibc release
-        run: cd example && make clean && make TARGET=aarch64-unknown-linux-gnu PROFILE=release
+        run: cd example && make clean && make LIBC=gnu LINK=dynamic PROFILE=release
       - name: build example -- glibc debug
-        run: cd example && make clean && make TARGET=aarch64-unknown-linux-gnu PROFILE=debug
+        run: cd example && make clean && make LIBC=gnu LINK=dynamic PROFILE=debug
 
   build-size-gnu-unstable:
     runs-on: ubuntu-latest
@@ -53,4 +53,5 @@ jobs:
         run: rustup toolchain install nightly -t aarch64-unknown-linux-gnu -c rust-src --no-self-update
 
       - name: build example -- glibc size-optimized
-        run: cd example && make clean && make TARGET=aarch64-unknown-linux-gnu PROFILE=size-optimized CARGO="rustup run nightly cargo" LTO=1
+        run: cd example && make clean
+          && make LIBC=gnu LINK=dynamic PROFILE=size-optimized CARGO="rustup run nightly cargo" LTO=1